Low activity and seasonal change in population size structure of grenadiers in oligotrophic abyssal North Pacific Ocean

Analysis of video recordings of swimming, in abyssal grenadiers Coryphaenoides. spp. revealed site differences in tail-beat frequencies. At the highly oligotiophic deep central North Pacific (CNP: 5800 m depth) station fishes had significantly lower tail-beat frequencies (0.73 +/- 0.02 Hz. mean +/- S.E.) than fishes of similar size at the shallower 'Station F' (Sta. F: 4400m depth) beneath the more productive waters of the California Current Upwelling (1.06 +/- 0.04 Hz). These behavioural differences may be evidence for the proposed physiological adaptations of Coryphaenoides armatus and Coryphaenoides yaquinae, to different depths and food supply levels. At CNP. smaller fishes (38.9cm mean L-T) were present in autumn than in summer, (59.4cm L-T) suggesting large-scale migrations across the abyssal ocean floor despite the observed slow swimming speeds. (C) 2003 The Fisheries Society of the British Isles.